Relay Board shorting

This is my first time posting on this site, so please be helpful and redirect me into the correct area if I have made a mistake. Thanks! I have searched and could not find anything similar to my issue.
I have an 05 350z set up for drifting. Full exhaust, welded diff, manual, tuned ECU, CAI, gutted, no A/C, etc.
Car drove totally fine for several weeks. Took it out to practice drifting, car handled like a champ. Took it out again after a couple weeks and after about five minutes, revved it up and let off the clutch during a dead stop - car just died.
Cranked but would not start. After several hours of diagnostics and replacing the fuel pump, realized there was a short in the relay board. We checked each relay (they all worked) and then grounded the fuel pump from the relay board directly to the battery and you could hear the fuel pump turning back on. There had to be a grounding issue inside the relay board behind the battery, we figured.
Ordered a used relay board online and after 5 minutes of installing - boom. Car turned right on. Went out for a test drive and all was fine. Drove nicely, even able to spin around a corner going slowly and then accelerate WOT. Perfect, no issues.
Decided to try it again. Revved it up to 4k, dropped the clutch and boom. Car died. Same problem. Cranks but does not start and you cannot hear the fuel pump turning on.

There has to be a short somewhere but the car has many modifications and I don't know where to start. I can replace anything mechanical on the car but I am mostly clueless when it comes to electrical/wiring. What am I even looking for? Anyone experienced this before? Are shorts more likely to happen in a certain spot? Can i splice the fuel pump into other wiring and bypass the short? Why can I accelerate WOT, but not from a dead stop?

ANY help or pointers in the right direction are greatly appreciated. Thank you guys so much!

The car still dies if under heavy load from a dead stop. I believe I have narrowed it down to a problem in the engine wiring harness.
If I replace the engine harness, can I use ANY wiring harness from a 2003-2005 350z? Does the trim model matter? Would it be better to have a custom wiring harness? If so, how much should I expect to pay?